1. Field of the Invention
The present invention relates to a die-expanded molding apparatus and method for synthetic resins, as well as die-expanded molded foam obtained thereby, and more particularly to a technique for improving the packing density of starting material beads in cavities to obtain molded products with more uniform packing density.
2. Description of the Related Art
As shown in FIG. 13, a die-expanded molding apparatus for forming molded products using starting material beads consisting of a thermoplastic synthetic resin comprises a pair of mutually opposed forming molds 100 and 101, and a packer 111 for packing starting material beads in a cavity 104 formed by the two forming molds 100 and 101, where chambers 102 and 103 are formed on the back side of the two forming molds 100 and 101, respectively, several vent holes 105 and 106 communicating between the cavity 104 and the chambers 102 and 103 are formed in the two chambers 100 and 101, respectively, and a service fluid such as steam, air, or cooling water needed for molding is fed to the chambers 102 and 103. In this case, upper service ports 107 and 108 are provided in the tops of chambers 102 and 103, respectively, to supply heated steam, and bottom service ports 109 and 110 connected to a vacuum pump or drain pipe are provided in the bottoms to supply steam to the cavity 104.
The several vent holes 105 and 106 penetrating through the forming molds 100 and 101 are actually provided when core vents, which consist of cylinders having lids with an outside diameter of 7 to 12 mm perforated by vent holes consisting of round holes of about 0.5 mm ø or slits of about 0.5 mm width, are fitted into core vent attachment holes arranged at a pitch of 20 to 50 mm in the forming molds 100 and 101.
When expanded foam is molded using such an expansion molding apparatus, the forming molds are first closed to form the cavity 104, pre-expanded starting material beads of polystyrene or the like are transported from a starting material tank (not shown) through the packer 111 into the cavity 104 and packed there, the starting material beads in the cavity 104 are then heated with hot steam, they are expanded and fused and are then cooled to solidification, and the forming molds 100 and 101 are opened to allow the expanded molded foam to be taken out.
One problem in particular which needs to be remedied in such a molding method, however, is the considerable difference between the packing density of the starting material beads at specific locations in the cavity 104 and the packing density at other locations. These specific locations can be broadly divided into (1) the interior of the cavity 104 where the detailed portions of molded products having complex shapes are formed, (2) the outer peripheral distal portion 104a of the cavity 104, and (3) the parts facing the packer 111 in the cavity 104.
Causes of this variation in packing density are described in detail in sections (1) through (3) below, but before that, the most commonly used method for packing starting material beads will be briefly described.
((1)) Cracked packing, ((2)) pressure packing, ((3)) compression packing, and the like are widely used methods for packing starting material beads.
(1) Cracked packing is employed when the air used during packing cannot be adequately expelled from the core vents alone, which are arranged in the core mold and cavity mold; during packing, the core mold and cavity mold are not completely closed, but are left open (are cracked) to an extent equal to about 10% of the floor thickness of the molded product, for example, to let the air used during packing to escape from the gap between the core mold and cavity mold.
(2) In pressure packing, the interior of the starting material tank holding the starting material beads is pressurized to between about 0.2 and 1.5 kg/cm2, while the cavity is left open to atmospheric pressure through the core vents and chambers, in which state the starting material beads are delivered into the cavity and packed there by means of the pressure differential between the starting material tank and the cavity.
(3) In compression packing, the pressure p in the starting material tank is pressurized to about 1.0 to 5.0 kg/cm2, which is higher than that in pressure packing, the interior of one chamber is pressurized, and the pressure differential (p−p1) of the pressure p1 in the cavity communicating through the vent holes is varied, so as to deliver and pack the starting material beads.
Causes of the variation in packing density are described below.
(1) Interior of Cavity for Forming Detailed Portions of Molded Products Having Complex Shapes
In the three aforementioned packing methods, suitable pressure differential is ultimately applied between the starting material tank and the cavity, and the starting material beads are delivered by the current of air produced on the basis of this pressure differential. In the cavity 104 having the relatively simple shape illustrated as an example in FIG. 13, the starting material beads are fully packed throughout, resulting in a shape with few local packing irregularities, so that a final expanded molded foam can be obtained with relatively uniform quality and few packing irregularities.
However, the results are different in the case of shapes with deep, narrow recesses 112 having a pouch-shaped cross section in the center plane of the core mold 101 such as that shown in FIG. 14(a) (in two locations above and below in FIG. 14) and in the case of shapes with deep, narrow recesses 112 having a pouch-shaped cross section in the center plane of the cavity mold 100 such as that shown in FIG. 14(a) (similarly in two locations above and below in FIG. 14). In these two cases, the current of air acting as the advancing force for packing the starting material beads settles in these portions, which makes it difficult for the starting material beads to be packed all the way into the interior of the recesses 112 or 113 having a pouch-shaped cross section and results in drawbacks such as extremely uneven packing or, in the worst cases, unsuccessful molding due to packing defects.
Efforts have been made to arrange special packers for recesses that are difficult to pack in order to remedy such problems, but they have resulted in the inconvenience of increasing the amount of air that is used, or in the need to reduce the number of molded foam articles which can be formed per mold, with the problem of considerably lowered productivity. That is because the number of packers that can be attached per molding apparatus is usually limited to a certain extent because of the volume of the starting material tank, the supply capacity of the pressurized air, and the like. 18 packers are attached in the most common apparatus, for example, with 3 packers set up per cavity when the molded product has a simple shape, whereas when 6 are needed for molded products having a more complex shape, even though there is room for 6 molded foam articles, only 3 can be molded, cutting productivity in half.
In addition, the increase in the amount of air supplied to the cavity per unit time when the number of packers is increased results in a sudden drop in the air pressure in the cavity immediately after packing and the like, and slows down the expulsion of air from the cavity, and the like, causing all the more variation in packing density. The number of packers used and the arrangement of the packers are thus a concern for designers of molds, and a great many elements require trial and error, making standardization extremely difficult to achieve in this field. The packing density tends to be lower particularly in parts that are some distance from the packer or in narrow, bottomed parts such as the recesses described above and the like, and it is necessary to increase the overall packing density in order to ensure a suitable packing density in such parts which are difficult to pack, resulting in a heavier molded foam than when the density is uniform.
Additionally, in terms of molding, it is necessary to further expand the starting material beads, and to increase the hot steam pressure, so that the starting material beads are thoroughly fused in parts with low packing density when the packed starting material beads are heated with steam. However, when the hot steam pressure is increased in parts with such low packing density, the parts with a high packing density become overheated, resulting in expansion pressure which is higher than that during normal molding. Thus, when the molded product is cooled, a longer time is needed to reduce the high expansion pressure to an expansion pressure allowing the molded product to be removed from the mold, and the longer molding cycle leads to a drop in productivity. Furthermore, the uneven expansion pressure in the various parts of the molded product during heating or cooling results in poor mold releasability and poor packing properties, and thus in the problems of lower productivity and yields.
(2) Outer Peripheral Distal End of Cavity
In pressure packing and compression packing, as shown in FIG. 13, the outer peripheral distal end 104a of the cavity 104 forms a dead end because the starting material beads are packed into the cavity 104 while the two molds 100 and 101 are completely closed. The current of air produced by the pressure differential between the starting material tank (not shown) and the cavity 104 thus settles in the outer peripheral distal end 104a, making it difficult to pack the starting material beads and tending to result in irregular density.
In cracked packing, on the other hand, the core mold and cavity mold are not completely closed, and are left open to an extent equal to about 10% of the floor thickness of the molded product, for example, so the outer peripheral distal end of the cavity does not form a dead end, but since the core mold and cavity mold are closed after being packed, the density in the floor portion of the molded product is higher than that in other portions to an extent corresponding to the cracked gap, resulting in a separate problem of irregular density.
Additionally, when the two molds are completely closed after being packed with the starting material beads in cracked packing, the outer peripheral distal end of the cavity forms a dead end, resulting in the following problems when the starting material beads are heated by steam and cooled by cooling water to remove the molded product from the mold.
When the starting material beads are heated, hot steam is supplied from one chamber to another, for example, allowing the hot steam to pass through the starting material beads in the cavity, but, as shown in FIG. 13, when the outer peripheral distal end 104a of the cavity forms a dead end, it becomes difficult for the hot steam to reach the outer peripheral distal ends 104a, and heat flows into the outer peripheral parts 100a and 101a of the mold where the heat volume is relatively high, making it difficult for the temperature of the starting material beads in the outer peripheral distal ends 104a to increase, with a correspondingly slower increase in temperature than in other parts, so that the heating process takes a longer time, resulting in the problem of a longer overall molding time.
When the molded product is cooled, cooling water is sprayed onto the molds 100 and 101 from nozzles (not shown) disposed in the chambers 102 and 103, the pressure in the chambers 102 and 103 is reduced to allow the water adhering to the molds or moisture in the cavity 104 to evaporate, and the expanded molded foam is cooled off along with the molds by the vaporization heat at that time, but now, in contrast to the heating time of the starting material beads described earlier, the outer peripheral distal end 104a of the cavity 104 becomes difficult to cool as a result of heat conduction from the outer peripheral portions 100a and 101a of the molds, and the time needed to sufficiently cool the parts of the expanded molded foam located at the outer peripheral distal end 104a of the cavity 104 is a problem.
When the molded product is released from the mold, the expanded molded foam is pushed out by an ejector pin (not shown) from the back side of the cavity mold 100 while the molds 100 and 101 are open, allowing the expanded molded foam to be taken out of the mold, but a problem that occurs when the core mold 101 is opened is that water which has collected in the inter-mold cavity 116 along the seams of the molding apparatus around the frames 114 and 115 and the outer peripheral portions 100a and 101a of the molds flows out, wetting the expanded molded foam which is the final product.
(3) Parts of the Cavity Facing the Packer
Although the packing density of the starting material beads in this part is high, conventional packing methods will be discussed in further detail before the relevant mechanism is discussed.
In the three packing methods noted above, the starting material beads in the packer 111 are returned to the starting material tank at the end of the packing operations, and a step referred to as blow back takes place, where the distal end of the packer 111 is closed.
Blow back is described with reference to FIG. 15. First, the packer 111 is composed of a packing pipe component 120, through which the starting material beads pass, and a flange 121. The tip is connected through a tube 102 to a supply hole 122 in the cavity mold 100. A packing tube 124 is connected between a starting material tank 123 and the packer 111, allowing the starting material beads to be delivered into the cavity 103 between the two molds.
When the pressure in the cavity 103 is adjusted to a negative pressure relative to the pressure in the starting material tank 123 with this arrangement, the starting material beads are delivered from the starting material tank 123 through the packing pipe 124 and the packing pipe component 120 of the packer 111 into the cavity 104 by the pressure differential, as shown in FIG. 15(a). In this case, pressurized packing air is introduced from the packing air valve 125 and is sprayed out from a spraying hole 126 in the distal end of the packing pipe component 120 as indicated by the arrows in FIG. 15(a) in order to urge the starting material beads in such a way that the delivered starting material beads smoothly enter the cavity 104 and are tightly packed there.
When the cavity 104 is thus filled with the delivered starting material beads, for example, in the manner depicted in FIG. 15(b), the packing air introduced from the packing air valve 125 is blocked by the tightly packed starting material beads and turns around in a U-turn from the spraying hole 126, resulting in a back flow through the packing pipe component 120. Due to this current of air, the starting material beads in the packing pipe component 120 are pushed back toward the packing pipe 124, leaving the component empty. This step in the blow back process is normally referred to as the natural blow back process.
The starting material beads in the packing pipe component 120 are ejected by the natural blow back, a plunger shaft 127 housed in the flange 121 is then pushed out, and the plunger tip 128 is pushed out, as shown in FIG. 15(c), so that the starting material bead supply hole 122 of the cavity mold 100 is closed off. The starting material beads are thus packed in a sealed state in the cavity 104. The starting material beads remaining in the packing pipe 124 are also blown back and are all returned to the starting material tank 123, ending the packing process.
In any type of conventional packing method, however, the parts of the expanded molded foam corresponding to the location of the aforementioned starting material bead supply hole 122 are subject to the phenomenon of over-packing, resulting in the problems of molded products with defective shapes and defective appearance.
The phenomenon of over-packing occurs because the starting material beads which are sent through the packing pipe component 120 of the packer 111 into the cavity 104 during natural blow back are believed to adhere to the cavity 104 side of the supply hole 122, as shown in FIG. 15(b), and the excess starting material beads collect into a protruding shape on the packer 111 side, but lumps of the excess starting material beads are pushed into the packed starting material beads by the plunger tip 128, as shown in FIG. 15(c), when the supply hole 122 is sealed off, presumably causing an abnormal increase in the density in that portion.
Although measures have been attempted to prevent the build up of lumps of excess starting material beads by adjusting the pressure, spraying angle, time, and the like of the packing air sprayed from the spraying hole 126 in order to prevent such over-packing, further problems that have resulted include a longer blow back treatment time, greater consumption of packing air, and the like, yet the effects on over-packing have still been unsatisfactory, and the problem remains to be solved.
An object of the present invention is to provide a die-expanded molding apparatus and method for synthetic resins allowing the packing density of the starting material beads in the aforementioned areas to be uniformly adjusted, as well as the die-expanded molded foam obtained thereby.